The present invention relates to a transmission/reception integrated radio-frequency (RF) apparatus used on a terminal side connected to, e.g. a two-way cable television (CATV) network.
With recent development in multi-media technology and digital technology in the fields of broadcasting and communications, amalgamation of broadcasting and communications technologies will shortly be realized. For example, a CATV line, which has a higher data transmission capacity than a telephone line, may be utilized to constitute a network, over which data communication services can be provided. In this case, a communication device (e.g. a personal computer), serving as a subscriber terminal, is connected to the CATV network. The personal computer, for instance, accesses a server via the CATV network and the Internet.
In this system, a unit called a cable modem, which functions as an interface with the CATV line, is disposed in the subscriber's house, and the user connects the cable modem to his/her PC. Thereby, the user can access an external network, such as the Internet, via a CATV broadcasting center. An example of a tuner of the cable modem described in Jpn. Pat. Appln. KOKAI Publication No. 11-103427, published Apr. 13, 1999.
In the CATV broadcasting, the center station sends broadcasting signals to each subscriber's house, using RF signals (downstream signals) of normally 90 MHz to 860 MHz. The receiving terminal, on the other hand, frequency-converts the RF signals once to three times to intermediate frequencies by means of a tuner in the cable modem, following which the intermediate frequencies are digitally demodulated. In addition, the receiving terminal can send information digitally demodulated by QPSK (Quadrature Phase Shift Keying) or 16 QAM (Quadrature Amplitude Modulation) from the individual terminal side to the center station, using RF signals (upstream signals) of normally 5 MHz to 65 MHz.
The level of the downstream signal is weak, and normally −15 to +15 dBmV, at the input terminal of the cable modem, while the level of the upstream signal is normally +8 to +58 dBmV at the output terminal of the cable modem.
The upstream signal output terminal of the cable modem is connected to an outdoor line via a coaxial cable. If overvoltage is applied due to indirect lightning stroke, semiconductor devices in the cable modem may be destroyed. As a method for preventing destruction due to overvoltage at the RF signal line, it is known to provide a protection circuit using a high-pass filter or a diode. However, the output level of the signal transmission circuit is high, and +58 dBmV at maximum. Consequently, a harmonic component (a second or third harmonic) may be produced by the protection circuit.
The upstream signal is controlled so as not to temporally overlap an upstream signal from some other user. For this purpose, a transmission time zone is assigned to the upstream signal transmission circuit, and the upstream signal transmission circuit is controlled and set in the non-operative state at time zones other than the assigned transmission time zone. If the upstream transmission circuit continues to send noise at a time zone other than the assigned one, the noise will be superimposed on the other user's upstream signal to which this time zone is assigned, and the center station cannot receive a correct signal.
An upstream signal and a downstream signal share a single coaxial cable. The upstream signal and downstream signal are thus frequency-separated by a diplexer built in the tuner of the cable modem. In the diplexer, it is needed isolation, to be ensured to prevent interference, between the upstream signal and downstream signal.
In addition, if the components of the cable modem are further reduced in size and the integration density of the IC is enhanced, it is also necessary to ensure isolation between the intermediate frequency of the downstream signal and the upstream signal.
Moreover, the cable modem simultaneously transmits the upstream signal and receives the downstream signal. Thus, the cable modem needs to be designed to stably receive the downstream signal both at the time of outputting and at the time of not outputting the upstream signal.
In the design of the cable modem, as described above, attention should be paid to the following problems.
(1) Since the level of the upstream signal is high, a harmonic component produced by the overvoltage protection circuit may interfere with the weak downstream signal. For example, when an upstream signal of 40 MHz is being transmitted, a third harmonic (120 MHz) of 40 MHz may interfere with a downstream signal of 120 MHz.
(2) If a certain user's cable modem continues to be in the active state over the assigned transmission time zone, noise may be sent to the cable and the center station may fail to exactly receive an upstream signal from some other user's cable modem.
(3) Where a circuit for transmitting an upstream signal (transmission signal) and a circuit for processing a downstream intermediate-frequency signal are integrated in an LSI for the purpose of reduction in size of circuit components, the upstream transmission signal may interfere with the downstream intermediate-frequency signal on a connection line between each circuit and an RF processing section.
(4) The upstream signal transmission circuit is activated/deactivated between the transmission time zone and the non-transmission time zone. With the activation/deactivation, a circuit current in the upstream signal transmission circuit may vary and a ripple may occur in the power supply voltage of a local oscillator which frequency-converts the downstream signal. If a ripple occurs in the power supply voltage of the local oscillator, side-band spuriousness may be caused and the quality of reception of the downstream signal may deteriorate.